Non-storage electron multiplier operation of transmission storage tube

ABSTRACT

A method and apparatus is described for providing non-storage electron multiplier operation of transmission storage tubes and other cathode ray tubes employing a transmission mesh target and a separate phosphor screen spaced therefrom. The improved operation is achieved by bombarding the dielectric layer on the mesh target electrode with low velocity flood electrons to charge such dielectric to a negative voltage with respect to the target electrode, thereafter bombarding the charged dielectric with a beam of high velocity writing electrons having energies greater than the first crossover potential to cause secondary electron emission at a greater than unity ratio to provide electron multiplication and attracting the secondary electrons through the mesh openings of the target electrode to the phosphor screen. The flood electron gun may be turned off during writing so that the flood electrons do not strike the phosphor screen in order to provide better image contrast. This electron multiplication increases the current of the electron beam striking the phosphor screen and thereby increases the maximum nonstored writing speed and the brightness of the light image.

United States Patent [191 Curtin 1 Mar. 19, 1974 [75] Inventor: Christopher John Curtin, Portland,

Oreg.

[73] Assignee: Tektronic, Inc., Beaverton, Oreg.

[22] Filed: Dec. 8, 1971 [21] Appl. No.: 206,073

[52] US. Cl 315/11, 315/12, 313/68 A [51] Int. Cl. H01j 31/48 [58] Field of Search 315/11, 12; 313/68 A, 83

[56] References-Cited UNITED STATES PATENTS 3.277.333 10/1966 Williams et a1. 315/12 3.470.414 9/1969 Helt et a1. 315/12 3.594.607 7/1971 Frankland. 315/12 3.611.000 10/1971 Johnston 315/12 3.710.173 l/l973 Hutchins et a1 315/12 3.717.786 2/1973 Massey et a1. 315/12 3,293,474 12/1966 Gibson. Jr 315/12 X 3.312.850 4/1967 McMillan. Jr. et a1. 313/68 A 3.317.782 5/1967 Anderson 315/11 3.339.099 8/1967 Anderson 313/68 A 3.413.513 11/1968 Donoghue et a1... 315/11 3.614.820 lO/1971 Morris 313/68 A X 3.633.064 H1972 Herman 315/12 3.649.866 3/1972 Salgo 315/11 OTHER PUBLICATIONS Selective Erasure and Nonstorage Writing In Direct-View I-Ialftone Storage Tubes, Lehrer, Proceedings of the IRE, 1961. pp. 567-573.

Primary ExaminerCarl D. Quarforth Assistant Examiner--P. A. Nelson Attorney. Agent. or Firml(larquist, Sparkman, Campbell, Leigh, Hall & Winston 5 7] ABSTRACT A method and apparatus is described for providing non-storage electron multiplier operation of transmission storage tubes and other cathode ray tubes employing a transmission mesh target and a separate phosphor screen spaced therefrom. The improved operation is achieved by bombarding the dielectric layer on the mesh target electrode with low velocity flood electrons to charge such dielectric to a negative voltage with respect to the target electrode, thereafter bombarding the charged dielectric with a beam of high velocity writing electrons having energies greater than the first crossover potential to cause secondary electron emission at a greater than unity ratio to provide electron multiplication and attracting the secondary electrons through the mesh openings of the target electrode to the phosphor screen. The flood electron gun may be turned off during writing so that the flood electrons do not strike the phosphor screen in order to 11 Claims, 2 Drawing Figures PATENTEDHAR 19 I974 FlG.l

o STORE -|O TO -HOV.

I lnlvlLlrl lllll WRITING GUN IE 5OV SWEEP FIG. 2

SECONDARY OV ELECTRONS WRIT ING BEAM NON-STORAGE ELECTRON MULTIPLIER OPERATION OF TRANSMISSION STORAGE TUBE BACKGROUND OF THE INVENTION The subject matter of the present invention relates generally to increasing the nonstored writing speed of cathode ray tubes containing transmission mesh targets by operating such target as an electron multiplier, and

in particular to an apparatus and method for operating a transmission storage tube so that its mesh storage target acts as an electron multiplier for the writingbeam to provide a faster nonstored writing speed and brighter light image. The present invention is particularly useful for displaying transient input signals of the single shot or nonrepetitive type, but may also be used to display a small number of successive input signals without repreparation of the target electrode.

The method and apparatus of the present invention provide a nonstore operation different from that of the storage operation of transmission storage tubes in that a high potential gradient, two or three times that of the storage potential gradient, is produced across the dielectric of the target so that the secondary electrons emitted from such dielectric upon bombardment by the writing'beam are attracted through the mesh openings of the target electrode to the phosphor screen and away from the collector mesh electrode positioned on the opposite side of such target from such screen. In addition, after charging the target dielectric to prepare it for writing, the flood gun is cut off during writing so that the light image on the phosphor screen is formed solely by the writing beam and the secondary electrons emitted from the storage dielectric due to bombardment of the high velocity writing electrons. Of course, in the storage operation of a transmission storage tube, the flood electrons are transmitted through the target mesh openings in those areas struck by the writing beam to produce the light image on the phosphor screen. The method and apparatus of the present invention also differs from the operation of a conventional cathode ray tube employing electron multiplier mesh targets in that such targets are not prepared for writing by bombarding them with low velocity of flood electrons to charge the dielectric and thereby produce a high potential gradient across such dielectric before the beam of high velocity writing electrons strikes such target.

SUMMARY OF INVENTION It is, therefore, the object of the present invention to provide an improved method and apparatus for operating a cathode ray tube including a transmission mesh target to achieve a faster nonstored writing speed.

Another object of the invention is to provide such a method and apparatus in which the mesh target is operated as an electron multiplier to give a faster nonstored writing speed and higher brightness light image.

Still another object of the invention is to provide such a method and apparatus in which low velocity flood electrons are caused to uniformly bombard the target dielectric to charge such dielectric to a negative voltage with respect to the target electrode and to produce a high potential gradient across the dielectric to prepare the target for writing so that the secondary electrons emitted by the dielectric upon bombardment by a high velocity writing'electron are attracted by the potential gradient through such target electrode to the phosphor screen.

A further object of the present invention is to provide such a method and apparatus in which the flood electrons are cut off or otherwise prevented from passing BRIEF DESCRIPTION OF DRAWINGS Other objects and advantages of the present invention will be apparent from the following description of a preferred embodiment thereof and from the attached drawings of which:

FIG. 1 is a schematic longitudinal section view of a transmission storage tube operated in accordance with the method and apparatus of the present invention; and

FIG. 2 is an enlarged section view of a portion of a tube of FIG. 1 schematically showing the nonstored electron multiplier operation of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT As shown in FIG. 1, one embodiment of the method and apparatus of the present invention includes a cathode ray tube containing a transmission mesh target 10 which may be the storage target of a transmission type storage tube. The transmission storage target includes a target mesh electrode 10, having a dielectric layer 12 of any suitable secondary electron emissive insulator material, such as a hard dense layer of magnesium oxide about two or three microns thick, provided on one side of such electrode facing a source 14 of high velocity writing electrons. The source may be an electron gun that focuses such writing electrons into a narrow writing electron beam 16. The writing beam 16 is emitted from a writing gun cathode 18 connected to a high negative DC. potential of about -1 500 volts. The current density of the writing beam is determined by the potential on a control grid 20 in front of cathode 18, which may be at about l550 volts. After passing through the conventional focusing and accelerating anodes of the electron gun 14, the writing beam 16 is deflected horizontally by a pair of horizontal deflection plates 19 and vertically by a pair ofvertical deflection plates 21 in order to write a charge image on the dielectric 12 of the target in a conventional manner. Thus,

when the tube of the present invention isemployed as.

the display device of a cathode ray oscilloscope, a horizontal sweep signal in the form of a linear ramp voltage is applied to the horizontal plates 19 while a vertical input signal is applied to the vertical plates 21 so that the writing beam is deflected to form a charge image on the target of the waveform of such vertical input signal.

A pair of flood guns, each including a flood gun cathode 22 and an anode 24 are provided in the tube for bombarding the dielectric layer 12 of the target substantially uniformly with low velocity flood electrons. The flood gun anode 24 is connected to a positive D.C. potential of about +50 volts. The flood gun cathodes 22 are connected by switch 26 to zero volts in the on position, or to +50 volts in the off" position. In the off position, the flood guns are cut off so that they do not emit any flood electrons, while in the on" position, such flood guns emit two broad defocused beams 28 of low velocity flood electrons. Three collimating electrodes 30,32 and 34 are provided as conductive bands coated on the inner wall of the tube envelope 36, and connected to D.C. potentials of +60, +75 and +90 volts, respectively, in order to collimate the flood electrons and cause them to strike the dielectric layer 12 at approximately right angles thereto, as well as to uniformly distribute such flood electrons over the sur face of such dielectric.

A collector mesh electrode 38 is positioned in front of the transmission storage target 10 and 12 and is connected to a source of positive D.C. potential of about +100 volts in order to collect secondary electrons emitted from the dielectric 12 during the storage operation of such target. However, the collector electrode only performs a collimating function on the flood electrons and does not act as a secondary electron collector during the non-storage electron multiplier operation of the present invention.

A phosphor screen 40 is provided within the tube on the opposite side of the target 10 from the writing gun 14. The phosphor screen includes a layer 40 of phosphor material provided on the inner surface of a glass face plate 42 which is sealed to the end of the funnel portion of the envelope 36 to form the evacuated envelope of the tube. A thin conductive layer 44 of aluminum or other suitable metal is provided over the phosphor layer 40 and connected to a high positive D.C. potential of about +7 kilovolts. The conductive layer 44 acts as an accelerating electrode and is sufficiently thin so as to be permeable to electrons striking such layer to enable them to excite the phosphor screen 40 after they are accelerated through the high field between such conductive layer and the target electrode 10. In addition, the conductive layer 44 reflects light emitted by the phosphor screen 40 to increase the brightness of the light image transmitted through face plate 42 in a conventional manner.

As shown in FIG. 1, the mesh target electrode 10 is connected by a switch 46 in its nonstore position to a positive D.C. potential of about +25 volts during the electron multiplier operation of the present invention or its *store position to a more negative potential of about lO to +10 volts during its storage operation. During the conventional storage operation of the transmission storage tube of FIG. 1, the writing beam 16 strikes the dielectric layer 12 and causes secondary electrons to be emitted therefrom which are collected by the collector electrode 38. These secondary electrons are not transmitted through the mesh electrode 10 to the phosphor screen due to the small potential gradient across the dielectric layer at this time. This causes the written areas of the dielectric layer 12 to be charged sufficiently positive so that the low velocity flood electrons 28 are transmitted through the mesh openings of the target in the written areas of the dielectric l2 and to produce a corresponding light image on the phosphor screen 40.

This stored operation differs from the nonstored electron multiplier operation of the present invention, hereafter described, which is made possible due to the high potential gradient produced across the dielectric when the switch 46 is in the nonstore position connecting the mesh electrode to a D.C. voltage source which is considerably more positive than that applied thereto in the store" position of switch 46. Thus, while the nonstore voltage is typically about +25 volts it may vary between approximately and +40 volts depending upon the thickness of the dielectric layer, the electrode spacing and other factors, but is always at least two times that of the most positive store voltage applied to the target electrode 10.

The electron multiplier operation of the present invention is schematically illustrated in FIG. 2 and is achieved when switch 46 is moved to the nonstore position to apply a positive D.C. voltage of about volts to the target mesh electrode 10. First, the transmission target is prepared for writing by uniformly bombarding the dielectric layer 12 with low velocity flood electrons 28 to charge the exposed outer surface of the dielectric layer 12 negatively to the zero volts potential of the flood gun cathode. Next, the charged dielectric is bombarded with the writing beam 16 of high velocity writing electrons to cause secondary electrons 48 to be emitted from such dielectric. The potential gradient across the dielectric layer 12 is of sufficient magnitude to attract the secondary electrons 48 through the mesh openings of the target electrode 10 toward the phosphor layer and away from the collector electrode 38. The secondary electrons are fur ther accelerated by the high field of the accelerating electrode 44 so they strike the phosphor screen and produce a light image of high brightness.

The writing electrons 16 strike the dielectric 12 with sufficient energy that they are above the first crossover voltage on the secondary electron emission curve of the dielectric and at a voltage, below the second crossover voltage, where the secondary emission ratio is greater than one. This means that one primary electron of the writing beam which strikes the dielectric 12, causes more than one secondary electron to be emitted by such dielectric and attracted through the mesh electrode 10 to the phosphor layer 40, so that the transmission target operates as an electron multiplier. Thus, the potential gradient across the dielectric layer 12 and the field of the accelerating electrode 44 are sufficient to overcome the field of the collector electrode 38 so that the secondary electrons 48 are attracted through the target electrode mesh openings toward the phosphor layer 40 and away from the collector electrode. Furthermore, it is preferable to prevent the flood electrons 28 from passing through the target mesh electrode 10 and striking the phosphor screen during the formation of the light image by the secondary electrons 48 in order to provide better contrast for the light image. This may be achieved by moving switch 26 from the on position to the off" position to cut off the flood gun before the writing beam 16 strikes the dielectric layer 12. As a result of the increased current of the electron beam striking the phosphor layer 40 due to the above described electron multiplication, the maximum nonstored writing speed and brightness of such tube are increased by a factor of at least two times that of conventional nonstored operations of such tubes. Thus, the previous nonstored operations of a transmission storage tube do not involve charging the storage dielectric or use the transmission target as an electron multiplier, but merely bombarded the phosphor screen directly with the writing beam like a conventional cathode ray tube.

It should be noted that any suitable secondary emissive insulator material may be employed for the dielectric layer 12 including magnesium oxide, potassium chloride, aluminum oxide, magnesium fluoride and silicon dioxide. They may be high density dielectrics formed by conventional vacuum deposition techniques, or highly porous dielectrics having a density less than about 5 percent of their bulk density such as disclosed in U. S. Pat. No. 3,213,316 ofGoetz et al. In one particular example, a dielectric layer 12 of hard, dense magnesium oxide having a thickness of about two to three microns was employed on a target electrode of electro-formed nickel mesh of about 250 lines per inch. The target was spaced from the collector electrode 38 and from the accelerating electrode 44, approximately the same diatance of about 3 to 4 millimeters.

As a result of the increased brightness of the light image 50 due to the electron multiplier operation, the maximum nonstored writing speed at which the writing beam 16 can be deflected and a corresponding light image seen or photographed, is increased considerably. This means that transient signals of higher frequency may now be viewed. It should be noted that this nonstored writing speed is considerably faster than the stored writing speed as well as the conventional nonstored writing speed of a transmission storage tube.

It will be obvious to those having ordinary skill in the art that many changes may be made in the details of the above-described preferred embodiment of the present invention without departing from the spirit of invention. For example, a photocathode can be employed as the source of the high velocity writing electrons rather than the writing gun 14. Furthermore, a scanning beam source of low velocity flood electrons can be employed rather than the glood guns shown. Therefore, the scope of the present invention should only be determined by the following claims.

I claim:

1. A method of operation of a cathode ray tube having a phosphor screen and a-separate electron transmission target including a mesh electrode and a dielectric layer provided on the mesh electrode without covering its mesh openings, to provide faster nonstored writing speed in which the improvement comprises:

charging the target dielectric substantially uniformly so that the exposed surface of said dielectric is charged to a negative voltage with respect to the potential of said mesh electrode; bombarding said charged dielectric with high velocity writing electrons to cause secondary electrons to be emitted by said dielectric, said writing electrons having energies above the first crossover potential of said dielectric so that its secondary electron emission ratio is greater than unity; and

attracting said secondary electrons through the openings of said mesh electrode to cause them to bombard the phosphor screen positioned on the opposite side of said target from the source of said writing electrons and spaced from said target to produce a light image on said screen so that said target operates as an electron multiplier.

2. A method in accordance with claim 1 in which the tube is a direct view transmission storage tube, said charging is achieved by bombardment of the dielectric with low velocity flood electrons and also includes preventing said flood electrons from being transmitted through said target mesh electrode during the production of said light image.

3. A method in accordance with claim 2 in which the source of said flood electrons is cut off when the writing electrons strike said dielectric.

4. A method in accordance with claim 1 in which the secondary electrons are accelerated through a high electrical field between said target and said phosphor screen.

5. A method in accordance with claim 1 in which the writing electrons are provided by an electron beam which is deflected to form the light image.

6. A method in accordance with claim 2 in which the tube is a transmission storage tube which also includes a collector mesh electrode positioned on the opposite side of said target from the phosphor screen, and the negative voltage on the surface of said charged dielectric produces asufficient voltage gradient across the dielectric layer to attract the secondary electrons through the target mesh electrode toward said screen and away from said collector electrode.

7. A cathode ray tube apparatus in which the improvement comprises:

an electron transmission target including a target. mesh electrode and a dielectric layer provided on said target electrode without covering its mesh openings;

charging means for charging the target dielectric substantially unifonnly so that the exposed surface of said dielectric is charged to a negative voltage with respect to the potential of said target electrode;

writing means for bombarding said charged dielectric with high velocity writing electrons to cause secondary electrons to be emitted by said dielectric, said writing electrons having energies above the first crossover potential of said dielectric so that its secondary electron emission ratio is greater than unity;

a phosphor screen provided on the opposite side of said target from said writing means and spaced from said target; and

field means for attracting said secondary electrons through the mesh openings of said target electrode to said phosphor screen to produce alight image on said screen so that said target operates as an electron multiplier.

8. An apparatus in accordance with claim 7 in which the charging means includes a flood gun means for bombarding the dielectric with low velocity flood electrons and also includes means for cutting off the flood gun means to prevent the emission of flood electrons during the formation of said light image.

9. An apparatus in accordance with claim 7 in which the writing means includes a writing gun for producing a beam of said writing electrons and deflection means for deflecting said beam horizontally and vertically to form the light image.

10. An apparatus in accordance with claim 7 in which the tube is a direct view transmission storage tube, the charging means is a flood gun means for bombarding the dielectric with low velocity flood electrons, and which also includes a collector mesh electrode, and

tor electrode.

11. An apparatus in accordance with claim 7 in which the tube is a direct view transmission storage tube and the charging means includes a flood gun means for bombarding the dielectric with low velocity electrons. 

1. A method of operation of a cathode ray tube having a phosphor screen and a separate electron transmission target including a mesh electrode and a dielectric layer provided on the mesh electrode without covering its mesh openings, to provide faster nonstored writing speed in which the improvement comprises: charging the target dielectric substantially uniformly so that the exposed surface of said dielectric is charged to a negative voltage with respect to the potential of said mesh electrode; bombarding said charged dielectric with high velocity writing electrons to cause secondary electrons to be emitted by said dielectric, said writing electrons having energies above the first crossover potential of said dielectric so that its secondary electron emission ratio is greater than unity; and attracting said secondary electrons through the openings of said mesh electrode to cause them to bombard the phosphor screen positioned on the opposite side of said target from the source of said writing electrons and spaced from said target to produce a light image on said screen so that said target operates as an electron multiplier.
 2. A method in accordance with claim 1 in which the tube is a direct view transmission storage tube, said charging is achieved by bombardment of the dielectric with low velocity flood electrons and also includes preventing said flood electrons from being transmitted through said target mesh electrode during the production of said light image.
 3. A method in accordance with claim 2 in which the source of said flood electrons is cut off when the writing electrons strike said dielectric.
 4. A method in accordance with claim 1 in which the secondary electrons are accelerated through a high electrical field between said target and said phosphor screen.
 5. A method in accordance with claim 1 in which the writing electrons are provided by an electron beam which is deflected to form the light image.
 6. A method in accordance with claim 2 in which the tube is a transmission storage tube which also includes a collector mesh electrode positioned on the opposite side of said target from the phosphor screen, and the negative voltage on the surface of said charged dielectric produces a sufficient voltage gradient across the dielectric layer to attract the secondary electrons through the target mesh electrode toward said screen and away from said collector electrode.
 7. A cathode ray tube apparatus in which the improvement comprises: an electron transmission target including a target mesh electrode and a dielectric layer provided on said target electrode without covering its mesh openings; charging means for charging the target dielectric substantially uniformly so that the exposed surface of said dielectric is charged to a negative voltage with respect to the potential of said target electrode; writing means for bombarding said charged dielectric with high velocity writing electrons to cause secondary electrons to be emitted by said dielectric, said writing electrons having energies above the first crossover potential of said dielectric so that its secondary electron emission ratio is greater than unity; a phosphor screen provided on the opposite side of said target from said writing means and spaced from said target; and field means for attracting said secondary electrons through the mesh openings of said target electrode to said phosphor screen to produce a light image on said screen so that said target operates as an electron multiplier.
 8. An apparatus in accordance with claim 7 in which the charging means includes a flood gun means for bombarding the dielectric with low velocity flood electrons and also includes means for cutting off the flood gun means to prevent the emission of flood electrons during the formation of said light image.
 9. An apparatus in accordance with claim 7 in which the writing means includes a writing gun for producing a beam of said writing electrons and deflection means for deflecting said beam horizontally and vertically to form the light image.
 10. An apparatus in accordance with claim 7 in which the tube is a direct view transmission storage tube, the charging means is a flood gun means for bombarding the dielectric with low velocity flood electrons, and which also includes a collector mesh electrode, and voltage means for providing said storage tube with a nonstorage mode of operation in which the flood electrons are prevented from being transmitted through the target mesh electrode to the phosphor screen during the formation of said light image, and the secondary electrons are attracted through said target mesh electrode to the phosphor screen and away from the collector electrode.
 11. An apparatus in accordance with claim 7 in which the tube is a direct view transmission storage tube and the charging means includes a flood gun means for bombarding the dielectric with low velocity electrons. 